Direct methanol fuel cells (DMFC) are widely known membrane electrochemical generators in which oxidation of an aqueous methanol solution occurs at the anode. As an alternative, other types of light alcohols such as ethanol, or other species that can be readily oxidized such as ethylene glycol, dimethyl ethylene glycol and oxalic acid, can also be used as the anode feed of a direct type fuel cell, and the catalyst of the invention can be also useful in these less common cases.
In comparison to other types of low temperature fuel cells, which generally oxidize hydrogen, pure or in admixture, at the anode compartment, DMFC are very attractive as they make use of a liquid fuel, which gives great advantages in terms of energy density and is much easier and quicker to load or refill. On the other hand, the electrooxidation of alcohol fuels is characterized by slow kinetics, and requires finely tailored catalysts to be carried out at current densities and potentials of practical interest. DMFC have a strong thermal limitation as they make use of an ion-exchange membrane as the electrolyte, and such component cannot withstand temperatures much higher than 100° C. which affects the kinetics of oxidation of methanol or other alcohol fuels in a negative way and to a great extent.
The quest for improving the anode catalysts has been ceaseless at least during the last twenty years. It is well known to those skilled in the art that the best catalytic materials for the oxidation of light alcohols are based on binary or ternary combinations of platinum and other noble metals. In particular, platinum-ruthenium binary alloys are largely preferred in terms of catalytic activity, and they have been used both as catalyst blacks and as supported catalyst, for example on active carbon, and in most of the cases incorporated into gas diffusion electrode structures suited to be coupled to ion-exchange membranes.
Platinum and ruthenium are, however, very difficult to combine into true alloys: the typical Pt:Ru 1:1 combination disclosed in the prior art almost invariably results in a partially alloyed mixture. The method for the production of binary combinations of platinum and ruthenium of the prior art starts typically from the co-deposition of colloidal particles of suitable compounds of the two metals on a carbon support, followed by chemical reduction. Co-deposition of platinum and ruthenium chlorides or sulfites followed by chemical reduction in aqueous or gaseous environment lies probably in the very different reactivity of the two metal precursors towards the reducing agents. The platinum complex is, in most cases, reduced much more quickly, and a phase separation of the two metal occurs before the conversion is completed. A platinum-rich alloy and a separate ruthenium phase are thus commonly observed.
Some methods did not include a step of deposition of precursor compounds onto the support. For example, U.S. Pat. No. 6,551,960 by Laine, et al. taught a method for depositing PtRu methanol reformation catalyst on activated carbon, metal, and metal oxide supports. In the described procedure, the solvent polyhydroxylic alcohols, such as ethylene glycol, glycerol, triethanolamine, or trihydroxymethylaminomethane was used as the reducing agent to react with dissolved Pt and Ru compounds, including chlorides, acetates, and acetylacetonate. Two severe factors can adversely influence the uniformity of the precursor compounds on the supports: (1) the process where two soluble species react to form a precipitate usually results in uncontrolled deposition process and the initially formed precipitates will act as nuclei centers for more precipitates to occur, so large particles and dendrite growth will occur; (2) Pt(acac)2 is much easier than Ru(acac)2 to reduce, so the formation of Pt and Ru metal particles will not be simultaneous and the resulting metal particles will be very poorly alloyed.
Outside catalyst or fuel cell area, there were some patent teachings about chemical vapor deposition to make metal or metal alloy film. For example, U.S. Pat. No. 6,303,809 taught about making Ru film and Pt/Ru film by chemical vapor depositions of selected compounds, such as those containing carbon monoxide and β-diketones, e.g., RC(O)CHC(O)R1 where R and R1 are alkyl groups. It was described that highly reflective, smooth, adhesive films are formed on the surface of Si wafer, Pyrex glass, and Al203. These films are not used as catalysts. Because of the process nature of this approach, only films containing very large particles, well above microns, can be formed.